Iranians on Friday carry the flag draped coffin of Mostafa Ahmadi Roshan,a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran, who was killed in a brazen daylight assassination Wednesday in Tehran.(AP)

Thousands gathered in the Iranian capital Tehran Friday to bury a top nuclear expert killed by a targeted bomb blast this week.

Iran has blamed the U.S. and Israel for the assassination and called for a UN investigation. While U.S. Secretary of State Hillary Clinton denied American involvement, Defense Secretary Leon Panetta said, “we have some idea as to who might be involved.”

The independent news site, irannuc.ir, is quoting an Iranian security official saying the country is preparing a covert counteroffensive against the West. “None of those who ordered or carried out these attacks should feel secure in any part of the world,” the official says.

The threat comes as tensions between Iran and the West reach a new high, with the U.S. leading an effort to impose sanctions on Iran over the country’s nuclear program, and Iran threatening to disrupt the flow of oil in the Strait of Hormuz.

Guest:

Vali Nasr, professor of international politics at Tufts Fletcher School and a fellow at the Brookings Institution. He was senior adviser to U.S. ambassador Richard Holbrooke on Pakistan and Afghanistan. He Tweets @vali_nasr.